The present invention relates to dual annular combustors such as those used in aircraft gas turbine engines. More particularly, the present invention pertains to an improvement in fuel delivery systems for dual annular combustors. By coordinating a three manifold fuel delivery system with a three position staging valve with fuel/air digital electronic control (FADEC), the present invention realizes improvements in reduced emission levels and improved durability and operability characteristics in dual annular combustors.
A schematic illustration of a prior art dual annular combustor is provided in FIG. 1. A combustor assembly 10 is housed in casing 12. Diffuser 14 delivers pressurized air to the combustor 16 which is provided with multi-hole liners 17. The holes in the liners provide a means for circulating air to cool the liners as the combustor becomes very hot during operation. Air passages 33A and 33B provide a path for the cooler air from the diffuser to circulate about the combustor.
The majority of compressed air from the diffuser enters the forward area of the combustor. This forward area of the combustor is comprised of a pilot dome 22 and a main dome 24 which are separated or divided by center body 34 to accommodate a pilot stage area 26 and a main stage area 28. Domes 22 and 24 are arranged in a double annular configuration wherein the two form the forward boundaries of what are separate, radially spaced, annular combustors, i.e., pilot stage area 26 and main stage area 28. A cowl is located to the front of center body 34 and further serves to demarcate the pilot dome from the main dome.
The domes are connected to the liners 17 by bolts or the like. Disposed in the domes 22 and 24 are carburetor devices 31A and 31B, respectively, which house fuel cups and swirlers. The fuel cups and swirlers are spaced in a circumferential manner at the front area of the combustor 16. The fuel cups of the pilot stage are located radially outward from the fuel cups of the main stage. Fuel is supplied to the fuel cups by means of fuel nozzle 19 which is comprised of dual nozzle inlets 20A and 20B. Nozzle inlet 20A supplies fuel through fuel nozzle stem 18 to pilot dome 22 and nozzle inlet 20B supplies fuel through fuel nozzle stem 18 to main dome 24.
FIG. 2 is a partial frontal, sectional schematic illustration taken along line A--A of FIG. 1. With reference to FIG. 2, fuel from the fuel stem 18 is directed into fuel injectors 21A and 21B and into fuel cups 37A and 37B. The fuel injectors 21A and 21B are slidably disposed in fuel cups 37A and 37B which, as has been mentioned, are housed at a plurality of circumferentially spaced locations around pilot dome 22 and main dome 24. Primary swirlers 30A and 30B are provided for each carburetor device in the pilot and main domes and surround the fuel injectors and fuel cups to provide swirled air which is mixed with the fuel. The products resulting from the combustion of fuel and air exit the pilot and main stage and expand through a downstream turbine section 36 (FIG. 1).
Over the years, environmental and competitive market place considerations have intensified the need to reduce the level of aircraft gas turbine engine emissions. The dual annular combustor offers an advantage over the traditional single annular combustor because the two separately fueled domes of the dual annular combustor allow a reduction in hydrocarbon (HC) and carbon monoxide (CO) emissions which are typically more prevalent at low power operation. Furthermore, the dual annular design achieves a reduction in oxides of nitrogen (NO.sub.x) and smoke emissions at high power operation.
The above-described characteristics are achieved by exclusively utilizing the pilot dome during times of low energy requirements such as start-up and idle and utilizing the main dome and pilot dome as further power is necessitated. The main dome receives more and more fuel as the power needs increase. The characteristics are further achieved by enriching the fuel air ratio (F/A) of the pilot dome which is fired during starting and low power operation, and leaning the F/A of the main dome during high power operation.
These opposing low power/high power F/A requirements for achieving lower emission levels are virtually impossible to attain with a fixed geometry, single annular design. For this reason, the single annular combustor design is a compromise between achieving low HC and CO emissions along with good starting and lean blowout operability characteristics at low power and achieving high NO.sub.x emissions, smoke and non-optimum pattern and profile factor at high power. The dual annular combustor concept allows optimization at both ends of the power spectrum.
A schematic of a conventional dual annular combustor fuel delivery system is presented in FIG. 3. Fuel control 40 directs fuel to modulating staging valve 42, which, when opened, allows fuel to be directed to main stage manifold 44. Valve 42 is modulated by positioning it in an "ON" or "OFF" position. When valve 42 is closed, fuel can only enter pilot stage manifold 46. Manifold 46 receives a measure of fuel whether the valve 42 is opened or closed. In conventional dual annular combustors, fueling of the main stage has been dependent on the position of the staging valve, such as staging valve 42, which is typically hydraulically controlled.
FIG. 4 provides a comparison of the dome swirler ratio (i.e., equivalence ratio) for a single annular combustor 50 and a conventional dual annular combustor 52 at varying levels of thrust and dome swirler ratios. The ratio is defined as the actual FAR divided by the stoichiometric FAR. The dome swirler ratio is the actual fuel to air ratio in a dome cup divided by the stoichiometric fuel to air ratio for the fuel used. The graph depicted in FIG. 4 illustrates a staged area of operation 48 and an unstaged area of operation 49. Staged area 48 corresponds to those levels of thrust in which the pilot stage of the combustor is engaged, only. After the thrust level has reached approximately 15%, the main stage begins to receive fuel in increased amounts as power requirements necessitate.
Indicated in FIG. 4 are the four engine operating points which are used to measure the overall emission characteristics of the engine. Operating point A is the 7% of thrust idle position, operating point B is the 30% of thrust approach position, operating point C is the 85% of thrust climb position, and operating point D is the 100% of thrust take-off position.
Superimposed on these data are the relative emission differences at the four engine operating points used to measure the overall emission characteristics of the engine (i.e., 7% idle, 30% approach, 85% climb, and 100% takeoff). With the current fuel delivery concept, significant improvements in emissions are achieved at all test points except 30% approach.
The differences in emission capability at the 30% of thrust as shown in FIG. 4 for a dual annular combustor having a conventional fuel delivery system is due to the selection of the transition point between staged (unfueled main stage) and unstaged (fueled main stage) combustion. To achieve a smooth and efficient transition which is externally non detectable, sufficient fuel must be delivered to the pilot and main stages at transition to prevent flameout of the pilot stage and to allow instantaneous ignition of the main stage. From this view point, the transition point should be set at the highest possible fuel air ratio (F/A) that will meet these requirements. On the other hand, selection of too high a transition F/A could result in hot section component life reduction due to the peaked outboard temperature profile generated by the pilot stage during staged combustion. From this viewpoint, the transition point between staged and unstaged combustion should be set at the lowest possible overall F/A. The resultant transition point is therefore a compromise between these two extremes. In the case of the system used heretofore, the optimum transition point is lower than the 30% approach power point. This results in an overall fuel air ratio at 30% approach that is leaner than desired.
Another drawback to the current two manifold dual annular fuel delivery system is that there is no practical way to fuel the main stage during start transients without flameout of the pilot stage. Flameout is defined as too little fuel to support combustion for a given amount of air. Fueling the main stage during the start transient is desirable since it would reduce the pilot stage peaked outboard exit temperature profile and reduce overall time to idle. The level of minimum fuel flow during starts is incompatible with dome air flows such that the resulting F/A's are not high enough to support combustion in the pilot stage and ignite fuel in the main stage.
Referring back to FIG. 4, line 52 represents the pilot stage of a conventional dual annular combustor during the staged operation of the system. During steady state staged operation, lower levels of HC and CO emissions occur with the dual annular combustor at line 52 than with the single annular combustor represented at line 50. Further, the dual annular combustor provides higher and therefore less desirable P&P (pattern and profile temperature factors) than the singular annular combustor at thrust levels less than 15%. (The P&P relates to the maximum temperature and its location existing at the combustor exit plane and therefore correlates with the life expectancy of engine components located in that vicinity.)
At thrust levels from 45% to 100%, the dual annular combustor represented by line 58 (pilot stage) and line 60 (main stage) achieves lower NO.sub.x and smoke levels when compared to the single annular combustor (line 50) while maintaining equivalent P&P. Further, the dual annular combustor provides higher combustor outlet temperature profile and pattern factors than the single annular combustor at thrust levels below 15%.
It is at thrust levels from 15% to 45% that the single annular combustor has proven superior in lowering HC and CO emissions over the conventional dual annular combustor. With a conventional dual annular system, significant improvements are made over the single annular design at all test points except that of the 30% of thrust approach position. Lines 56 and 54 represent the pilot stage and the main stage, respectively, of a conventional dual annular combustor at thrust levels of 15% to 45%. HC and CO emission levels for the conventional dual annular combustor are greater than that of the single annular combustor (line 50) for this level of thrust. Since approach thrust is an important emission rating point, an increase in HC and CO production at this rating point negatively impacts the achievable amount of emission reduction for the dual annular combustor.
Therefore, a need exists for a dual annular fuel delivery system which achieves a significant improvement in emissions of hydrocarbons and carbon monoxide at the 30% thrust position. Further, a need exists for a dual annular combustor which can fuel the main stage during the start to idle transient without flameout, while providing an acceptable combustor outlet temperature profile and pattern factor, and which reduces the time interval from start to idle power. Also, a need exists to enable partial staging of the combustor during the start to idle transient when fuel flow rates are high in order to improve the combustor outlet temperature profile, while returning to pilot operation only once the idle speed is reached thus achieving low HC and CO emissions.